This invention relates in general to a thick ambipolar layer on a photoresponsive device simultaneously capable of charge generation and charge transport.
In the art of electrophotography, an electrophotographic plate comprising a photoconductive layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles, and frequently over many thousands of consecutive cycles. There is also a great current need for long surface life, and flexible photoreceptors in compact imaging machines that employ small diameter support rollers for photoreceptor belt system compressed into a very confined space. Small diameter support rollers are also highly desirable for simple reliable copy paper stripping systems which utilize the beam strength of the copy paper to automatically remove copy paper sheets from the surface of a photoreceptor belt after toner image transfer. However, small diameter rollers, e.g., less than about 0.75 inch (19 millimeter) diameter, raise the threshold of mechanical performance criteria for photoreceptors to such a high level that spontaneous photoreceptor belt material failure becomes a frequent event for flexible belt photoreceptors. Still further, such criteria for mechanical performance may cause the crystallization or deterioration of small molecule materials within the polymer binders.
One type of single layered photoreceptor that has been employed in electrophotographic imaging systems comprises a conductive substrate and a single charge generating and transporting layer. The charge generating and transporting layer often comprises a chalcogenide material which is photoactive and unipolar. The expression "unipolar" means that the material transports a single sign of charge. In order to imagewise discharge a surface charge on the layer, frequencies of light are used which are highly absorbed in the chalcogenide material and therefore do not penetrate into the bulk of the layer. Thus, the region of the material near the surface acts as a charge generating layer and the bulk of the material acts as a charge transporting layer for one sign of charge. Yet another single layered photoreceptor that has been employed in electrophotographic imaging systems comprises a conductive substrate and a charge transfer complex consisting of poly(vinyl carbazole) and 2,4,7-tri-nitro-9-fluorenone.
One problem associated with unipolar single layer electrophotographic imaging members is that charges which are generated in the bulk of the material of a polarity opposite to that transported by the member become trapped. These trapped charges are known to cause several problems in electrophotographic applications, such as increased background in images and cyclic instabilities for machines which run several thousand cycles. Another problem with previous charge transfer complex single layer photoreceptors is their unfavorable environmental impacts and safety issues.
One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a charge blocking layer, a charge generating layer and a charge transporting layer. The charge transporting layer often comprises an activating small molecule dispersed or dissolved in a polymeric film forming binder. Generally, the polymeric film forming binder in the transporting layer is electrically inactive by itself and becomes electrically active when it contains the activating molecule. The expression "electrically active" means that the material is capable of supporting the injection of photogenerated charge carriers from the material in the charge generating layer and is capable of allowing the transport of these charge carriers through the electrically active layer in order to discharge a surface charge on the active layer. The multilayered type of photoreceptor may also comprise additional layers such as an anti-curl backing layer, an adhesive layer, and an overcoating layer.
One problem associated with multilayered electrophotographic imaging members is delamination. Since the various layers of a multilayered imaging member contain different materials, the adhesion of those materials will vary. In addition, greater time and cost factors are involved in the manufacturing of a multilayered electrophotographic imaging member, as well as a greater probability of imperfections due to the multiple layers.
U.S. Pat. No. 4,983,481 to Yu, assigned to Xerox Corporation, discloses a photoreceptor in which the charge generating and charge transporting functions are clearly separated into two layers. The charge generating layer is a thin layer (less than 2 micrometers) and the charge transporting layer is a thick layer (greater than 15 micrometers). Positive carrier (hole) transport in the thin charge generating layer is carried out by selenium and a small organic molecule (namely N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1-biphenyl-4,4-diamine. The negative carrier (electron) transport in the thin charge generating layer is also performed by the selenium. The disclosed photoreceptor is not ambipolar as only holes are transported in the thick charge transport layer.
U.S. Pat. No. 4,415,640 to Goto et al., assigned to Konishiroku Photo Industry Co., Ltd., discloses a single layered charge generating/charge transporting light sensitive device. Hydrazone compounds, such as unsubstituted fluorenone hydrazone, may be used as a carrier-transport material mixed with a carrier-generating material to make a two-phase composition light sensitive layer. The hydrazone compounds are hole transporting materials but do not transport electrons, such that the device is not ambipolar.
U.S. Pat. No. 4,552,822 to Kazmaier et al., assigned to Xerox Corporation, discloses charge generation and charge transport substances which are located in separate layers. A fluorenylidene malonitrile derivative is employed for the electron transporting substance such that the charge transport layer is an electron transporting layer, not a hole transporting layer, such that the device is not ambipolar. The (4-n-butoxycarbonyl-9-fluorenylidene)malonitrile is utilized in a layer separate from the charge generator layer such that there is no combined thick charge generating/charge transporting layer.
U.S. Pat. No. 4,559,287 to McAneney et al., assigned to Xerox Corporation, discloses a photoresponsive imaging member comprising a photogenerating layer having a photogenerating pigment optionally dispersed in an inactive resinous binder, an electron transporting layer, and a stabilizing amount of an arylamine electron donating compound. The electron transporting layer may contain a fluorenylidene derivative. The disclosed device does not have a combined charge generating/charge transport layer and the positively charged device contains a thick charge transport layer which transports electrons, but not holes, such that the device is not ambipolar.
U.S. Pat. No. 4,474,865 to Ong et al., assigned to Xerox Corporation, discloses a photoresponsive device comprising a supporting substrate and a photogenerating layer. The photogenerating layer is in contact with an electron transporting layer comprising a fluorenylidene derivative. The photogenerating layer contains photogenerating pigments dispersed in an inactive resinous binder composition. A process of preparing (4-n-butoxycarbonyl-9-fluorenylidene) malonitrile is explained in Example 1. No combined charge generating/charge transporting layer is disclosed, and the device is capable of transporting electrons in the charge transporting layer in the positive charging mode only, and thus is not ambipolar.
Though the above-mentioned references provide for a number of alternatives for electrophotographic imaging, there continues to be a need for a single layer electrophotographic imaging member, the single layer simultaneously being capable of charge generation and charge transport, and wherein the imaging member is ambipolar and operable in either positive or negative charging modes.